1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is capable of reducing a reset interval of a panel to increment a lighting time of a back light.
2. Discussion of the Related Art
Generally, an active matrix liquid crystal display (LCD) controls the light transmissivity of liquid crystal cells using an electric field to display a picture. To this end, the active matrix LCD includes a liquid crystal panel having liquid crystal cells arranged in a matrix type, and a driving circuit for driving the liquid crystal panel. The liquid crystal panel is provided with pixel electrodes for applying an electric field to each liquid crystal cell and a reference electrode (i.e., common electrode). A pixel electrode is formed at a lower substrate for each liquid crystal cell, while the common electrode is integrally formed at the entire surface of an upper substrate. Each pixel electrode is connected, via source and drain terminals of a thin film transistor using as a switching device, to a one of a plurality of data lines. Each gate terminal of the thin film transistors is connected to a one of a plurality of gate lines allowing a pixel voltage signal to be applied to pixel electrodes for one line.
Such an LCD makes use of red (R), green (G) and blue (B) color filters or color back lights to control a mixed ratio of the three original colors properly, thereby realizing a desired color. More specifically, an LCD using the color filters employs red, green and blue color filters for each pixel, including three liquid crystal cells, to realize a color by red, green and blue data applied simultaneously. An LCD using the color backlights turns on red, green and blue backlights sequentially in compliance with color data to be displayed. A color realization method for an LCD using such color backlights has been disclosed in Korean Patent Application No. P95-2771, filed on Feb. 15, 1995.
As shown in FIG. 1, the color LCD disclosed in the above Korean Patent Application charges any one of red, green and blue color data into liquid crystal cells in one vertical synchronizing interval (1 Vsync), and turns on the corresponding color back light at a middle time point of a color data charge time Tt, thereby expressing a color. To assure a sufficient lamp turn-on time to improve the brightness, the back light should be turned on before a charge of any one-color data into all of the liquid crystal cells in the liquid crystal panel has been completed. However, if the back light lamp is turned on before a charge of any one-color data into all the liquid crystal cells has been completed, then color purity is deteriorated, producing a color-blurring phenomenon.
For instance, in the case of charging green (G) data in the liquid crystal cells line-sequentially from the first line assuming that color data should be displayed in a sequence of red (R), green (G) and blue (B) colors as shown in FIG. 1, green (G) data has been charged in the upper liquid crystal cells at a time when the green (G) back light is turned on; while red (R) data from the previous frame has been charged in the lower liquid crystal cells in which green (G) data has not yet been charged. If the green back light is turned on in this state, then the upper liquid crystal cells charged with green (G) data expresses a normal color, whereas the lower liquid crystal cells still holding red (R) data from the previous frame results in a transmission of a green light to generate a color blur.
In order to prevent such a color-blurring phenomenon, all the liquid crystal cells are reset after displaying any one-color data and before displaying the next color data. More specifically, red (R) data voltage having been held in the liquid crystal cells is discharged after displaying red (R) data and before displaying green (G) data to reset all of the pixels before charging green (G) data. Since the backlight has been turned off during the majority of such a reset interval, as a reset interval becomes longer, a quantity of light transmitted through the panel becomes smaller. Thus, the total brightness is reduced.
However, the conventional reset method of the liquid crystal panel requires a relatively large time of 3.1 ms because a reset voltage is applied to the data line while scanning the gate line sequentially in similarity to charging the pixel data to thereby reset the liquid crystal cells. Accordingly, the backlight has been turned off during a charging time (i.e., 3.1 ms) of data plus a reset time (i.e., 5 ms), that is, during the maximum 8.1 ms in one vertical period of 16.67 ms, so that the brightness is reduced. Also, in the conventional reset method, power consumption is increased because the gate line is sequentially scanned twice (i.e., once for charge and once for reset) during one vertical period. In addition, since the liquid crystal cells in the panel are discharged to a voltage allowing no transmission of light in the reset interval, as the reset interval becomes longer, a time interval when the panel takes on a black color is lengthened to generate a flicker phenomenon that alternates a bright state and a dark state of a screen. As a result, since it becomes difficult to express a natural picture on the screen due to a relatively long reset interval, the conventional reset method fails to express a clear picture.
Recently, there has been suggested a scheme of allowing the red, green, and blue data to be sequentially displayed for one frame by increasing a charging speed of color data into the liquid crystal cells, because it is difficult to express a natural picture when any one color data is displayed in one frame. In this scheme, since a turn-on time of the back light is relatively shortened, it can avoid deepening the above-mentioned problems involved in the reset interval.